1. Field of the Invention
The present invention relates to a torque detecting apparatus suitable for a power steering apparatus of an automobile and the like. More specifically, the present invention relates to a torque detecting apparatus comprising a first shaft and a second shaft coaxially connected to each other via a connecting rod, a permanent magnet fixed to the first shaft, a plurality of soft magnetic materials fixed to the second shaft so as to be disposed within a magnetic field of the permanent magnet thus to form a magnetic circuit, and a sensor for detecting a magnetic flux induced by the magnetic circuit formed by the soft magnetic materials, so that when a torque is applied to the first shaft or the second shaft the torque is detected based on an output of the sensor.
2. Description of the Related Art
Many vehicles are today provided with a power steering apparatus that provides an assisting steering force by an electric motor, thus to relieve a load imposed on the driver in the steering operation. The power steering apparatus comprises an input shaft connected to a steering member (steering wheel), an output shaft connected to a wheel to be steered via, for example, a rack and pinion, and a connecting rod for connecting the input shaft and the output shaft. A steering torque applied to the input shaft is detected by a torque detecting apparatus according to an amount of torsion (specifically an angle) generated on the connecting rod, so that the electric motor for steering assistance interlocked with the output shaft is driven based on the detected steering torque value. In such a torque detecting apparatus of a power steering apparatus, a magnetic detection resolver that detects a rotational position with a coil, or an optical encoder that detects a rotational position by sensing light transmission has been conventionally employed.
Apart from the above, Japanese Patent Application Laid-Open No.2003-149062 proposes a torque sensor (torque detecting apparatus) having a configuration according to a vertical cross-sectional view shown in FIG. 1, which is different from the conventional ones. This torque sensor (torque detecting apparatus) comprises an input shaft 1 and an output shaft 2 coaxially connected to each other via a torsion bar (not shown), a ring-shaped 24-pole permanent magnet 15 fixed to the input shaft 1, two magnetic yokes 4c, 4d respectively constituted of a soft magnetic material, fixed to the output shaft 2 so as to be disposed within a magnetic field of the permanent magnet 15 thus to form a magnetic circuit, two flux condensing rings 8c, 8d magnetically connected to the magnetic yokes 4c, 4d respectively to induce the magnetic flux from the magnetic yokes 4c, 4d, flux condensing portions 19c, 19d respectively provided on the flux condensing rings 8c, 8d so as to condense a magnetic flux induced by the flux condensing rings 8c, 8d, respectively, and a magnetic sensor (Hall IC) 6 that detects a magnetic flux condensed on the flux condensing portions 19c, 19d. With such a torque sensor (torque detecting apparatus), a torque applied to the input shaft 1 can be detected based on an output of the magnetic sensor 6.
In the torque detecting apparatus disclosed in Japanese Patent Application Laid-Open No.2003-149062, the magnetic yokes 4c, 4d have to be disposed so as to receive a certain amount of magnetic flux from the permanent magnet 15. In other words, a certain area has to be ensured to allow the magnetic yokes 4c, 4d and the permanent magnet 15 to oppose each other. This, however, inevitably requires that the permanent magnet 15 has a certain length (height) in an axial direction of the input shaft 1, which inhibits reducing an overall dimension of the torque detecting apparatus in the axial direction of the input shaft 1, thus preventing making the torque detecting apparatus more compact.